Annular dies are used to form laminated products from thermoplastic melts (hereinafter "melt"). Conventional annular dies consist of a monolithic structure containing a core mandrel that may contain a spiral groove and axially stacked annular cylinders that concentrically surround the mandrel. Gaps between the mandrel and the innermost cylinder as well as gaps between the innermost cylinder and the next cylinder radially adjacent to it form passages for melt flow. Thus, an increase in the number of melt passages in a conventional die requires an increase in the radial thickness of the die structure. Melt can be supplied to the die both axially and radially through multiple entry openings. Each melt stream then flows axially through the die along the passages, and eventually joins into layers where these passages join within the die, and then finally the melt exits the die at the die lip and is blown into film. The spiral groove that can be provided on the outer surface of the mandrel can assist in controlling both the direction and rate of melt flow.
Several problems have been encountered with the conventional annular die design including deterioration of plastic melt, difficulties in controlling and adjusting temperature of the melt, and inability to maintain uniform film thickness. For example, melt residues may remain in the passages formed by the gaps between the mandrel and the cylinders as well as those between cylinders. These residues may contaminate fresh melt flow and thus deteriorate the quality of films made from the die containing such residues. In addition, removing such residues is extremely labor intensive, particularly because of the small size of the passages and the difficulty of taking the die apart. Thus, melt that stagnates in the die passages shortens the useful life of the die. Furthermore, conventional extruder design dictates that each melt delivered to a die have a separate extruder, thus multiple extruders take up vast amounts of space and clutter work areas.
In addition, it is difficult to produce a laminate product containing film layers made of different materials because the varied temperature requirements of different materials are difficult to meet. Multiple streams of melts are used to produce a laminate product, and each stream may require a different temperature depending on the properties of the melt material in the stream. For example, one melt may have a higher melting point or different thermal properties than another melt flowing within the die. Since the annular cylinders are concentrically arranged in the conventional die, it is difficult to control the temperature of the different melts axially flowing along the cylinders, because temperature is controlled by applying heat in a radial direction from the outer periphery of the die. Since the number of concentric cylinders surrounding the core is increased to form multi-layered films, the peripheral heating system makes it difficult to apply the proper heat to plastic melt that is flowing along the inner cylinders of the die.
Furthermore, the axial height resulting from the monolithic die design may produce inconsistent film thickness. Producing a laminate with an increased number of laminate layers using a conventional die not only requires more concentric cylinders that increases radial thickness but also increased axial height to join the melt passages within the die. Inconsistencies in the thickness of extruded film may result because increased axial height makes the die susceptible to thermal expansion, leading to inclining of the die.
U.S. Pat. No. 5,076,776 issued to Yamada et al. discloses an annular coextrusion die for a lamination product. The die consists of stacked annular plate-like rings with one opening in the center of the ring. Each plate-like ring has a number of manifolds cut into it that spiral inward. In operation, melt flows through an entry flow area adjacent to the center opening in each plate-like ring. A gap exists between the manifolds, and the melt overflows this gap to the next manifold. The center opening of the plate-like rings form a gap with the core mandrel creating an axial melt passage. The melt is thereby directed from the melt opening on the radial periphery of the plate-like ring through the manifolds, across the entry flow area into the melt passage and out through the die lip.
The die disclosed in Yamada et al. still requires labor-intensive die manufacture to produce the spiral manifolds of various thickness. In addition, the manifolds that are cut into plate-like rings require that these rings have a tangible thickness, which contributes to the overall axial height of the die. Furthermore, the melt residue problem associated with the conventional annular die remains an issue with this die design.